Annealed Co thin films: Pit formation and magnetic anisotropy

We present a study of the morphological and magnetic transformation of hcp( 0001) Co thin films, grown via sputtering on Al2O3(11 (2) over bar 0), whic h occurs when they are annealed in vacuum in the temperature range of 500 d egrees C < T-A< 600 degrees C. In this temperature range, the films undergo a reentrant smoothening transition. For very thin films (thickness t < 20 nm), this coincides with a fcc(111) to fcc(001) recrystallization, and with the appearance of rectangular pits. For thicker films, the recrystallizati on does not occur, even though the surface smoothens out leaving hexagonal pits behind. The number of pits per unit area decreases exponentially as th e thickness of the film is increased, demonstrating that interactions at th e film surface and film-substrate interface cause the smoothening and pit f ormation during the annealing process. Whereas the hcp(0001) and fcc(111) p hases are magnetically isotropic in the film's plane, the fcc(001) films ex hibit a biaxial in-plane anisotropy. (C) 2000 American Institute of Physics . [S0021-8979(00)50808-1].